1. Field of the Invention
The invention relates to a vehicular power transmission and, more particularly, to a hydraulic servo device for a clutch of the power transmission.
2. Description of the Related Art
In a vehicular power transmission system, a hydraulic servo for applying a clutch to connect rotary elements to each other is generally mounted in one of the rotary elements. The technology of the so-called "stationary cylinder type construction", in which the hydraulic servo is arranged in the case of the power transmission system, is disclosed in the prior art in Japanese Patent Laid-Open No. 119761/1995. When the stationary cylinder type construction is adopted, the push force of the hydraulic servo cannot be received, unlike the prior art hydraulic servo mounted in the rotary element, by a reaction member on the rotary element having the servo to constitute a closed loop of force in the rotary element thereby to establish an unbalance force. In this technique, therefore, there is adopted the construction in which the push force of the hydraulic servo is transmitted through the reaction member to the case.
In the vehicular power transmission system, generally speaking, a helical gear having a tooth trace inclined with respect to the axis, is used to smoothen the power transmission through the meshing portion of the gear. The helical gear establishes a thrust force at the time of transferring the power, but the acting direction of the thrust force changes with the running state of the vehicle. Specifically, the thrust force is inverted between the state in which the vehicle is driven by the power of the engine (to be called the driving time), i.e., when the throttle is ON, and the state in which the engine is driven by the wheels (to be called the coasting time) by the coasting of the vehicle, i.e., when the throttle is OFF. When this state change occurs, the controllability of the hydraulic servo is lowered for the following reasons when the construction is of the stationary cylinder type as in the prior art.
A predetermined gap is formed, between the individual members of the power transmission system, to allow for manufacturing error, thermal expansion or other similar factors. When the thrust force acts against the push force of the hydraulic servo, the reaction member moves by the gap so as to reduce the clearance in the application direction of the clutch thereby to reduce the ineffective stroke of the piston. When the thrust force acts in the same direction as that of the push force of the hydraulic servo, on the other hand, the clearance of the clutch is not reduced so that the ineffective stroke is elongated. Thus, when the oil pressure is supplied to the hydraulic servo so as to apply the clutch at the driving time and at the coasting time, the ineffective stroke time changes to lower the controllability.